1. Field of the Invention
The present invention relates to an apparatus for direct or indirect application of a liquid or pasty coating medium onto a traveling material web, notably of paper or cardboard.
2. Description of the Related Art
Apparatuses for direct or indirect application of a liquid or pasty coating medium onto a traveling fiber material web are used in so-called coating systems for providing a traveling material web, formed for example of paper, cardboard or a textile material, on one or both sides with one or several layers of the liquid or pasty coating medium, for example color, starch, impregnating fluid or the like.
In the so-called direct application, the liquid or pasty coating medium is applied by an applicator system directly onto the surface of the traveling fiber material web, the latter being carried during application on a rotating countersurface, for example an endless belt or a backing roll. In the indirect application of the medium, the liquid or pasty coating medium is first applied onto a substrate, for example the surface of a backing roll configured as an applicator roll. The coating medium is then transferred from the applicator roll to the fiber material web in a nip through which the fiber material web passes.
Employed for the above applicational purpose, e.g., are applicators whose metering system is equipped with an open-jet nozzle. As generally known, these applicators allow easy and relatively uncomplicated operation and the parts are not subject to a high level of wear. Therefore, the applicators are relatively maintenance-friendly and are normally used for web travel speeds up to 1500 meters/min. They are especially suited for papers sensitive to traction, for example, thin papers, where the risk of web break is relatively high.
On the other hand, when a coating system is to be operated at a still higher speed (e.g., above 1500 m/min), when a special depth of penetration is desired or an especially light coating weight is to be applied onto the fiber material web, applicators can be used which apply the liquid or pasty medium by way of a coating chamber formed between a doctor element (blade, bar or roll doctor) and a back-up strip. The liquid or pasty medium is usually kept at a certain pressure in this coating chamber.
Known from the German utility model DE-GM 84 14 904.3 is an applicator as described above for direct application of a liquid or pasty medium onto a traveling material web, notably of paper or cardboard, including a metering system and, coordinated with it, a rotating backing roll which supports the traveling fiber material web during application. The metering system includes a metering doctor mounted in a doctor bed and a back-up strip arranged a distance from the metering doctor in a direction opposite to the direction of rotation of the backing roll. The back-up strip bounds together with the metering doctor and the backing roll surface a chamber which is supplied with the medium by way of a feed slot. Usually provided on the metering system is a run-off surface across which the medium applied at surplus and issuing out of the coating chamber over the back-up strip drains under the effect of gravity. The medium is collected in a collection system and subsequently recycled to a medium circulation.
Equipped with coating chambers, these conventional applicators, however, have considerable disadvantages. For example, the ratio of applied to surplus, i.e., overflow medium quantity, is approximately 1:10. Consequently, a considerable medium quantity needs to be circulated in the operation of the applicator, requiring large and efficient pumps, filters, pipelines, control organs and, thus, a high overall energy demand of the applicator resulting in high operating costs. Therefore, such applicators are no longer competitive from an economic view. Furthermore, it has also been found that the coating quality achieved with these applicators is poor.
Known from German Patent Application No. P 44 44 779.5, assigned to the assignee of the present invention, is an apparatus for direct or indirect application of at least one liquid or pasty medium onto a traveling material web, notably of paper or cardboard, including at least one applicator for the medium. Coordinated with the applicator is a backing roll for receiving the liquid medium in the indirect application or for carrying the fiber material web in the direct application. The applicator includes a coating doctor as well as a cleaning doctor arranged a distance from the coating doctor in a direction opposite to the direction of rotation of the backing roll. The cleaning doctor bounds with the coating doctor a coating chamber for the medium. The applicator is equipped with openings or conduits through which the liquid or pasty medium issues out of the coating chamber in a direction substantially opposite to the direction of rotation of the roll. The medium impinges at least partly outside the coating chamber on the surface area of the backing roll, or the fiber material web, bordering on the cleaning doctor. The cleaning doctor features a doctor bar supported by a blade-like holder equipped with openings or conduits serving to enable rinsing or prewetting with the medium for cleaning purposes.
The aforementioned prior art's very unfavorable ratio between applied and surplus or overflow medium quantity has already been improved considerably with this type of applicator. The quality of the coating produced has also been boosted due to the self-cleaning effect of the cleaning doctor bar. Realizing these positive coating results, however, involves considerable equipment and engineering expense. In view of the manufacturing costs of the entire apparatus, therefore, it would be desirable to further simplify the applicator design while maintaining, or even improving, the coating results.